In recent years, data traffic amount of internet goes on increasing due to popularization of video distribution, spread of usage of data center, and so on, and capacity of a photonic network which enables the above has been increased. In local area network (LAN), standardization of 100 Gbps Eathernet (registered trademark) (GbE) which enables speeding up from 10 giga bit/s (Gbps) was completed in June 2010. In 100 GbE, four optical signals of 25.8 Gbps whose wavelengths are different are used while being wavelength division multiplexing. However, there are problems in a currently used 100 GbE optical module such that a size and a power consumption are large, and an optical module which is small in size and low in power consumption has been argued as an approach for further popularization.
In optical communication, an optical signal is generated by using a semiconductor laser, and this optical signal is transmitted by an optical fiber. A method to generate the optical signal is as follows:                A method modulating non-modulated light which is generated at a semiconductor laser by an external modulator.        A method modulating non-modulated light which is generated at a semiconductor laser by an optical modulator, using a modulator-integrated semiconductor laser where the semiconductor laser and the optical modulator are monolithic integrated.        A method directly modulating a current being applied to the semiconductor laser, namely a direct modulation method.        Among them, the direct modulation method is one in which a structure of an optical signal generation device (transmission device) is simple and a drive circuit of the optical signal generation device is also simple because the optical modulator is not necessary for the generation of the optical signal. Therefore, the direct modulation method is superior in a point of small-sizing of the optical module compared to other methods each requiring the optical modulator. In addition, further small-sizing can be expected by replacing the direct modulation laser which oscillates at different four wavelengths into a monolithic integrated array laser.        
Structures of the semiconductor laser are roughly divided into two types of a buried hetero (BH) structure and a ridge waveguide structure. The BH structure is illustrated in FIG. 1A, and the ridge waveguide structure is illustrated in FIG. 1B.
In the BH structure as illustrated in FIG. 1A, a mesa structure is formed on an n-InP substrate 101. N-InP, an active layer material, p-InP, a contact layer material are grown on the n-InP substrate 101, and they are etched to a middle of n-InP, to thereby form a groove 102. The mesa structure having an active layer 104, an upper cladding layer 105, and a contact layer 106 is formed on a lower cladding layer 103 by the formation of the groove 102. For example, InP being a semi-insulating semiconductor where Fe is doped is buried in the groove 102 by regrowth, and a high-resistance buried layer 107 is formed. A p-type electrode 108 is formed on the contact layer 106, and an n-type electrode 109 is formed at a rear surface of the n-InP substrate 111, respectively.
As illustrated in FIG. 1B, the ridge waveguide structure is one in which a lower cladding layer 112 and an active layer 113 are sequentially formed at a whole surface on an n-InP substrate 111, and a ridge part is formed on the active layer 113. P-InP and a contact layer material are grown on the active layer 113, and they are etched to a middle of the p-InP to thereby form grooves 114. The ridge part having an upper cladding layer 115 and a contact layer 116 is formed on the active layer 113 by the formation of the grooves 114. A p-type electrode 117 is formed on the contact layer 116, and an n-type electrode 118 is formed on a rear surface of the n-InP substrate 101, respectively.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2000-312051
[Patent Document 2] Japanese Laid-open Patent Publication No. 2011-233829
[Patent Document 3] Japanese Laid-open Patent Publication No. H6-232099
In the BH structure, the active layer 104 is completely buried in the high-resistance buried layer 107 being a semiconductor in a hetero structure, and therefore, it is possible to simultaneously perform confinement of light and carrier. In the BH structure, the carrier is effectively injected into the active layer 104, and therefore, it is an effective structure to reduce a volume of an active layer of a semiconductor laser. However, it is necessary to evenly bury the mesa structure whose bight exceeds 3 μm into the high-resistance buried layer 107, and therefore, manufacturing thereof is not easy, and various technologies are required.
On the other hand, the ridge waveguide structure is a structure in which a protruding-state ridge part is formed at an upper part of the active layer 113, and thereby, an equivalent refractive index at the upper part becomes large compared to a periphery of the upper part to thereby waveguide the light. In this structure, it is not necessary to perform the regrowth after the formation of the ridge part, and manufacturing thereof is easier compared to the BH structure. In the ridge waveguide structure, the active layer 113 exists at the whole surface, an active layer region which contributes to a laser oscillation is limited by a current injection region which is limited by the ridge part. However, actually, the injected carrier spreads at the upper cladding layer 115 on the active layer 113, and therefore, there is a defect in which an effective width of active layer becomes wide than a width of the ridge part. Besides, the active layer 113 exists continuously in a plane, and therefore, the injected carrier spreads in a horizontal direction, and there also is a defect in which a reactive current which does not contribute to the laser oscillation increases. Further, when an array laser is manufactured in the ridge waveguide structure by way of trial, the upper cladding layer 115 exists at the whole surface on the active layer 113, and the upper cladding layers 115 of respective semiconductor lasers are connected. Accordingly, there is a defect in which an electrical resistance between the semiconductor lasers becomes small, and signal deterioration due to an electrical crosstalk occurs.
As a structure to limit the current injection region by surrounding the active layer region existing directly under the ridge part with a semi-insulating layer, it is proposed to perform a process enabling high-resistance by means of impurity diffusion up to the active layer (refer to Patent Literature 1). However, by the impurity diffusion, an impurity concentration becomes maximum in a vicinity of a surface, and the impurity concentration decreases in a depth direction, and therefore, it is impossible to freely set the concentration and a diffusion length. Besides, as for InP, it is possible to change n-type InP into p-type InP or semi-insulation InP, but it is difficult to change p-type InP into n-type InP or high-resistance InP by ion-implantation or the impurity diffusion, and therefore, there is a problem in which an art of Patent Literature 1 is not applicable for a widely used structure on the n-type InP substrate where an upper part of the active layer becomes a p-type cladding.